In recent years, high frequency heating devices have been widely employed in various kinds of industries which require heating sources. Particularly, in the art of welding, hardening and the like, a high frequency current is used to heat a substance to be processed.
Such a high frequency heating device comprises, substantially, a high frequency power supply unit, a heat generating unit for producing the heat by a high frequency current supplied from the high frequency power supply unit, a matching unit for matching the high frequency current supplied from the high frequency power supply to a current to be supplied to the heat generating unit, and a current supply cable for connecting the high frequency power supply unit to the heat generating unit by way of the matching unit.
The current supply cable is a relatively important portion of the high frequency heating device for obtaining high efficiency and good heating characteristics. It is, accordingly, necessary for the current supply cable to be able to pass a current density of 50-100 A/mm.sup.2 at a potential of 10-20 KV and a frequency from 10 to several hundred KHz. Moreover, the current supply cable, in order to follow movement of the heat generating member, must be capable of flexing within a range of roughly 50-500 mm in the three directions, vertically, laterally and longitudinally. Furthermore, it is necessary to provide cooling means to prevent overheating of the current supply cable and so forth caused by the skin effect and the proximity effect attendant on high frequency current flows.
FIGS. 1 to 5 show various examples of conventional current supply cables.
The power supply cable 1, shown in FIG. 1, comprises a first conductor in the form of a copper plate 2 having one end fixed to a high frequency power supply unit (not shown in the drawings), a second conductor 3 to be electrically connected to a heat generating unit such as a work coil (not shown in the drawing), a flexible junction cable 4 which is formed by laminating a plurality of relatively thin conductive plates made of copper or an alloy of copper of thickness about 0.2-0.5 mm, a fastening means consisting of a plurality of bolts 5 for connecting the cable 4 to the first conductor 2 and the second conductor 3, and a hose 6 for providing cooling water.
The cable 4 can be bent in many directions, but has the disadvantage of being subjected to damage caused by cracks developed under torion when the second conductor 3 is moved in an oblique direction.
A problem in using such a flexible junction cable 4 is that the flexible junction cable 4 is relatively wide (100 through 600 mm) in proportion to the current capacity and will require extra space for air cooling purposes to prevent the surface of the junction cable 4 from being overheated by the skin effect and proximity effect produced when a high frequency alternating current flows.
There is another problem in a high frequency heating device using the flexible junction cable 4 in that it has been customary to cool part of both the first conductor 2 and the second conductor 3 by means of water (or other cooling medium) fed through the first conductor 2 in order to protect these parts from being overheated through the skin effect and proximity effect produced when high frequency currents flow through such conductors, and thus an additional hose is required in parallel with the flexible junction cable 4 in order to feed water to the second conductor 3, whereby more space is required and the cost increases.
FIGS. 2 and 3 show another example of a prior art current supply cable. The cable shown in FIG. 2 comprises a first conductor in the form of an electric conductive tube 7 having a flange 7a at its extreme end portion, a second conductor in the form of an electric conductive tube 8 having a connecting flange 8a, and a flexible junction conductor in the form of a corrugated metallic tube 9 having connecting flanges 9a and 9b. As is shown in FIG. 2, the tube 7 is connected in a liquid-tight fashion to the corrugated tube 9 by the flanges 7a and 9a, and second metallic tube 8 is also connected to the corrugated metallic tube 9 by the flanges 8a and 9b in a liquid-tight fashion.
The corrugated metallic tube 9 in such a construction as shown in FIG. 2 provides a means for passing a cooling medium such as water and electricity simultaneously. The metallic tube 9 is disadvantageously susceptible to damage by cracks developed under torsion in spite of its longer life for unidirectional movement of expansion and contraction.
A problem of the high frequency heating device using such a corrugated metallic tube is its lack of flexibility and limited range of movement necessitating the provision of bendable portions in the flexible corrugated metallic tube, thereby resulting in unnecessary space occupied, a more complex mechanism and larger electrical loss.
Irregular high frequency vibrations due to vortexes in the water passing across the recessed portions of the corrugated metallic tube can cause an accident due to water leakage from a part damaged by fatigue.
The difficulty of manufacturing such a corrugated metallic tube because of its considerably large diameter required to pass high-density currents represents another problem.
There is still another problem in a high frequency heating device using such a corrugated metallic tube.
This problem will be described with reference to FIG. 3. It has been customary to insert the end portion of the corrugated metallic tube 9 into a connecting portion 9c of the flange 9a and join them with a brazing metal 11. When the corrugated metallic tube 9 is forced to bend, the end portions of the connecting member 10 are subjected to maximum stress so that the breakage of the connecting member 10 tends to start from those portions.
In order to reinforce the junction of the flange and corrugated metallic tube to avoid breakage in the junction, it is possible for the flexible current supply cable to be coated with a braided wire and integrally brazed with the connecting part of the flange. In this case, the braided wire itself will inevitably be cooled by air only.
Therefore, the braided wire may burn out because high frequency alternating currents tend to flow through the skin portion of the braided wire.
Another type of current supply cable whose cross section is shown in FIG. 4 has a stiff sprial copper element 12a shaped like a coil spring, a plurality of twisted wires 12 arranged along the circumference of the stiff sprial copper element, a tape 13 wound around the twisted wires for restricting the movement thereof, a rubber sheath 14 covering the tape, and a water passage 15 provided in the central cavity formed by the stiff spiral copper element 12a. Such a current supply cable may be used in the case where an alternating current below 10 KHz is used, because such a current has a current density of substantially 10 to 20 A/mm.sup.2.
If the frequency rises as high as 400 KHz, however, the alternating current will tend to be deflected into section 16 shown by the hatched lines of FIG. 5 because it is a general practice for two current transmitting passages to be provided in parallel with each other to increase the efficiency of mutual inductance. The extremely high current density of substantially 50 to 100 A/mm.sup.2 and the construction of the flexible current supply cable in which the interior is cooled only by a cooling medium such as water can cause overheating in the section of twisted wires followed by lowering the mechanical strength against its bending and finally burning out the current supply cable. Parts in FIG. 5 with functions similar to parts in FIG. 4 are given the same numbers with primes.
There is another problem in a high frequency heating device using such a current supply cable in that in order to prevent the current supply cable from being deformed, the total thickness of the section including the twisted wires and rubber sheath is made large so that the total weight of the current supply cable increases and it becomes impossible to reduce the maximum allowable radius of bending curvature.